1. Field of the Invention
The present invention relates to a claw pole type synchronous motor and more particularly to a claw pole type stepping motor.
2. Description of the Prior Art
A claw pole type synchronous motor including a PM type two-phase stepping motor has been used widely because it has an excellent feature that it can be manufactured at a low cost. One of typical conventional claw pole type stepping motors is disclosed in Takashi Kenjo: "Stepping motors and their microprocessor controls", page 40 to 43, Oxford University Press, 1986 (Reprint).
A two-phase stepping motor which is a claw pole type synchronous motor has two stator assemblies comprising an A-phase stator assembly and a B-phase stator assembly having the same structure and coaxially arranged back-to-back, and claw poles formed circumferentially at equal intervals on each stator assembly.
In the conventional stepping motor of this type, the magnetized rotor is moved axially thereof every time the coils are excited. Since the length of the rotor is substantially equal to the distance between the upper surface of the upper stator assembly and the lower surface of the lower stator assembly, the axial movement induces magnetic variations between the rotor and the rotor assemblies. This produces adverse vibration of the motor.
In order to reduce vibration in the conventional motor, the claw poles are indexed on the A-phase and B-phase stator assemblies and are arranged back-to-back so that the separation angles between the center lines of the adjacent claw poles of both stator assemblies (i.e., the relative angle defined between the claw poles of the A-phase and B-phase) are made as accurately as equal as possible to the mechanical angle of 360.degree./N(N being the number of steps) which is equivalent to an electric angle equal to N/4 times the corresponding mechanical angle.
In spite of such strict control of the separation angles, vibration is generated every time the motor moves by one step. Further, the reduction of the vibration is limited due to the fact that ripples are large because the sum of squares of the wave form of the BEMFs (back-electromotive forces) is not linear. This limitation has been compensated to some extent by use of a frequency reduction device other than the motor.
In the course of recent advancement of the optical technology, recording devices which perform high density recording have been developed. Although having an excellent feature that the motor can be manufactured at a low cost, the conventional claw pole type stepping motor cannot have been used on such a highly efficient recording device, since occurrence of vibration in the motor must be strictly prevented.